Absorption refrigeration machines are widely used in commercial operations. A typical absorption refrigeration machine includes four major sections: absorber, generator, condenser, and evaporator.
In the absorber section, an absorbent fluid, typically an aqueous alkali metal halide solution, such as a lithium bromide solution, absorbs a refrigerant, typically water vapor. The absorber can be operated under sub-atmospheric pressure. The resultant weak or diluted absorbent fluid (about 40-58% concentration of alkali metal halide rich in the refrigerant) is pumped to the generator. Here heat is applied to the fluid to partially boil off the refrigerant to again concentrate the absorbent fluid. The concentrated absorbent solution (now about 63-65% alkali metal halide) from the generator is passed through a heat exchanger and then sprayed back into the absorber section where it resumes absorption of the refrigerant vapor.
The refrigerant vapor liberated in the generator migrates to the condenser where it is liquefied by exchanging heat with a cooling fluid (typically water) flowing through tubing (typically copper based alloy tubes) present in the condenser section. The liquid refrigerant in the condenser moves into the evaporator, which is also operated at an extremely low pressure. In the evaporator, the refrigerant cools the relatively warm system water circulating through the tubes of the tube bundle, and the chilled water is circulated to the load. Heat from the system water vaporizes the refrigerant water which then migrates to the absorber section for absorption into the concentrated solution and completes the cycle.
Aqueous alkali metal halide solutions are widely used as absorption fluids in commercial absorption refrigeration systems. An exemplary alkali metal halide solution for this application is a lithium bromide solution, adjusted to pH range of 7-13 with lithium hydroxide. Although this and other types of absorption solutions can be advantageous for the refrigeration cycles, alkali metal halides can be corrosive towards the materials used to construct the refrigeration machine. Such materials can include mild and stainless steel for containment components and copper or copper-nickel alloys for tube bundles, among others.
In addition to the surface damage caused by corrosion, the corrosion reaction evolves hydrogen gas as a byproduct. Incondensibles in the form of atoms or ions can easily enter and diffuse into metals, resulting in the degradation of their mechanical properties under certain system conditions.
The severity of corrosion can vary, depending upon factors such as temperature of the system, concentration of alkali metal halide in the absorption solution, metals used in the construction of the refrigeration unit, the presence of air, and the like. For example, during use, the internal temperatures of such machines can be high, typically up to about 450.degree. F. and higher, depending on the type of the absorption cycle, which can increase the corrosive effect of the alkali metal halide solution.
Various additives, such as lithium chromate, lithium nitrate, and lithium molybdate, have been proposed as corrosion inhibitors in alkali metal halide absorption solutions. However, lithium chromate can raise environmental concerns, and its use is being phased out. Further, the level of chromate and its oxidation state must be carefully maintained. If too little chromate is used, then it does not properly passivate the whole metal surface and pitting can result. Lithium nitrate can potentially evolve ammonium, which can cause stress corrosion cracking of copper based alloys such as heat exchanger tubes. Lithium molybdate exhibits only limited solubility in alkali metal halide solutions. In addition, lithium molybdate is metastable in aqueous halide solutions and thus it can be difficult to maintain a constant concentration of molybdate ions in solution.